It is well known in the art that it is a desirable processing property for polymer resins in meltblown processing to have a low viscosity when molten. For many commercial end-users, the melt-flow characteristics of standard, commercial polymer resins are not suitable because of their relatively high molecular weight, which results in a high melt viscosity. As low melt viscosity is desired, prior art references have sought to achieve low melt viscosity of polymer through controlled scission of the polymer chain. This controlled scission, in effect, reduces the post-reactor average molecular weight of the polymer chains. As the average molecular weight is reduced, the melt viscosity is lowered. Furthermore, the molecular weight distribution (MWD) is significantly altered.
It is well-known that polymer resins suitable for meltblown processing may be produced by preparing polymer with a relatively high melt viscosity and then subjecting it to a post-reactor molecular weight alteration using a chemical prodegradant, typically a free radical initiator, such as a peroxide. This degradation treatment occurs under such conditions that the melt viscosity of the polymer decreases to a specific value. However, when producing a pelletized polymer for future processing, this process has presented problems. U.S. Pat. Nos. 4,451,589; 4,897,452 and 5,594,074 all report that when peroxide treatment is used to produce a low melt viscosity polymer in a extrusion process, the resulting polymer is not easily pelletized. Specifically, the degraded polymer on exiting the extruder becomes so fluid and soft that it is difficult or impossible to cut into pellet form.
To avoid this problem, several processing techniques have used a degradation process involving a primary degradation wherein the average molecular weight of the polymer is reduced to a value above that desired for meltblown processing. The degradation is performed in an extruder wherein an additional amount of prodegradant remains impregnated in the pelletized polymer for further degradation. The additional prodegradant acts to further reduce the average molecular weight to the desired value during meltblown processing. U.S. Pat. No. 5,594,074 to Hwo, et al, U.S. Pat. No. 4,451,589 to Morman, et al and U.S. Pat. No. 4,897,452 to Berrier, et al all describe processes for making polymer pellets containing an unreacted free radical initiator. Using the impregnated free radical initiator, the polymers can be further degraded upon thermal treatment to form an ultra low melt viscosity polyolefin. U.S. Pat. No. 4,897,452 describes a process for the manufacture of propylene homopolymer or copolymer pellets in the presence of a primary and secondary free radical initiator, wherein the half-life of the second free radical initiator is at least twenty times longer than that of the first free radical initiator. In that invention at least 80% by weight of the second free radical initiator, and not more than 20% by weight of first free radical initiator remain intact in the pellets and available for subsequent decomposition during the conversion of the pellets into finished articles.
Another method consists of higher melt viscosity reactor granules that are coated with peroxide so that they crack to lower melt viscosity during meltblown processing. In all cases, the un-reacted peroxide cracks the polymer to low melt viscosity during meltblown processing.
However, materials having prodegradants either impregnated into or coated onto pelletized polymer have some disadvantages. In particular, there is a danger that the residual prodegradant within the polymer will react early, either before it gets to the end-user or before the end-user processes it. As a result, various lots of the polymer material may behave with a degree of inconsistency.
It is also known to produce polymer having a low melt viscosity directly from an in-reactor process. In this case no post-reactor molecular weight alteration is required, as the desired melt viscosity property is produced directly by the in-reactor polymerization of the monomer. A draw back of resins produced by in-reactor processes is that they are supplied in a flake rather than pellet form, resulting in the presence of a significant amount of powdery fines, which create difficulties in handling and transporting the material. Finally, in-reactor processing is not a viable option for a number of meltblown fabric processors that lack the particular conveying systems necessary to transport materials supplied in flake form.
Therefore, it would be desirable to provide a process for producing a polymer resin that has low melt viscosity and good melt flow in meltblown processing. A polymer resin produced by such a process would have a low melt viscosity, as measured by melt flow index, in combination with a low residual content of prodegradant. Such material would be provided fully or nearly fully reacted prior to melt blown processing. Such material would also be provided in a pellet form for easy handling and transport.